Abstract This project pursues the fundamental nature of the relationship between the early ocular vasculature (blood vessels and blood supply) and the developing neural retina. Although much is known regarding disorders related to the pathological persistence of the hyaloid vasculature in mammals, and pathologies involving the retinal vasculature, the developmental importance of the early ocular vasculature for the developing structures of the eye remains unknown and unstudied. Filling this knowledge gap will have tremendous impact. Abnormalities of the ocular vasculature are critical mediators of pathological progression in numerous retinal diseases. Access to a blood supply, and specific roles for endothelial cells are known to exist within other developing systems, and results from a small number of published developmental gain-of- function studies (those which increase vessel development) suggest a role for the early ocular vasculature in regulating development of the adjacent neural retina. In addition, studies using co-culture models suggest that direct cellular contact of neural progenitors with endothelial cells influenced neural stem cell proliferation and retinal cell differentiation. Our published and preliminary data provide further evidence that a regulatory relationship exists and is required in the developing eye in vivo, such that the early ocular vasculature must be present for normal retinal neurogenesis, and this requirement is not strictly metabolic. We now propose to discover the specific role of vascular endothelial cells in the control of retinal neurogenesis. Our objective in this application is to determine the cellular and molecular mechanisms underlying retinal abnormalities in embryonic zebrafish models for vascular dysgenesis. The embryonic zebrafish is ideal for these studies because the tissues are not yet dependent upon circulating hemoglobin for oxygenation, thus allowing innovative experimental manipulations of the vasculature that uncover non-metabolic, developmental roles for the vasculature. We propose to test the hypothesis that factors derived from the endothelial cells of the early ocular vasculature are necessary for normal retinal neurogenesis, by pursuit of two Specific Aims. 1. Define the roles of circulating vs. local endothelial cell-derived factors in regulating retinal neurogenesis and gliogenesis. 2. Identify regulatory targets of endothelial-derived factors. This project will lay the groundwork for future studies aimed at identifying endothelial- derived and circulation-derived factors and understanding their roles in retinal stem cell niches in embryonic eyes, regions of persistent neurogenesis, during retinal regeneration, and in vascular pathologies of the retina.